1. Field of the Invention
The present invention generally relates to a process for making a leather-like sheet, and to a leather-like sheet made thereby. More particularly, the invention relates to a manufacturing process thereof by impregnating a fibrous substrate with a polyurethane emulsion followed by coagulation. The leather-like sheet obtainable with the present invention exhibits a significantly improved softness and fulfillment feeling, a superior hand touch and physical properties extremely close to natural leather as compared with the conventional leather-like sheet obtainable in the prior art.
2. Description of the Prior Art
Artificial leather-like sheets have been commonly manufactured by impregnating fibrous substrates with resinous material, such as polyurethane as a bonding material. Typical examples of these manufacturing processes include: (a) a wet coagulation process in which fibrous substrate is impregnated with resin solution such as solution of polyurethane dissolved into an organic solvent such as dimethylformamide, and subsequently coagulated in non-solvent media, such as water; and (b) a dry coagulation process in which fibrous substrate is impregnated with resin solution such as polyurethane solution in an organic solvent, or with emulsion dispersed in water, and subsequently dried.
As for a wet coagulation process, Japanese Patent Publication Sho 45-33797 discloses a manufacturing process for making a sheet material using a silicone resin-treated fibrous substrate which is impregnated with a polymer solution or an emulsion based on polyurethane elastomer followed by coagulation. The wet coagulation process can provide better hand touch of natural leather compared with the dry coagulation process, but is disadvantageous in that it is inferior in productivity, and requires the use of an organic solvent such as dimethylformamide.
In the dry coagulation process, the use of an organic solvent can be avoided by using an emulsion. However, the use of emulsion tends to give significantly inferior hand touch and fulfillment feeling compared with the wet coagulation process. In the dry coagulation process, during drying stage resin tends to exist locally in the fibrous substrate thereby bonding fibers tightly at localized sites, and causing loss of softness of the sheet. This is evident from FIG. 1 which shows an electron micrograph of a cross section of a conventional leather-like sheet obtained by impregnating fibrous substrate with a general-purpose emulsion. In the conventional dry coagulation process with a resin emulsion, when the amount of resin to be stuck onto the fibers is reduced not to lose softness, the hand of fibrous substrate itself, such as nonwoven fabric appears, thereby failing to provide the hand touch of natural leather. On the other hand, when it is tried to obtain enough fulfillment feeling to provide the hand touch of natural leather, softness of the sheet will have to be sacrificed. In both the cases, it is impossible to obtain a highly desirable quality of hand touch of natural leather.
In the dry coagulation process using an emulsion resin, some consideration is given to add a softening agent after the resin impregnation step to provide desired softness. However, this requires an additional step of adding the same, which will lower the productivity, and is unlikely to afford the desired high-grade hand touch of natural leather.
On the other hand, when microfine fibers are used in the fibrous substrate, a more natural hand touch similar to those of natural leather can be obtained. Therefore, this technique is now being used to manufacture a high quality suede-like artificial leather. The followings are some typical examples of manufacturing processes hitherto used for the above.
(1) First, a resin such as polyurethane is added to a microfine fiber forming fibrous substrate of sea-island type, which is composed of composite spun fibers or mixed spun fibers. After the addition of the resin, the sea component is dissolved away or decomposed with an organic solvent or aqueous alkaline solution to leave the island component remaining as microfine fibers. PA1 (2) Fibrous substrate is made in advance by using microfine fibers already prepared, and then impregnated with a resin such as polyurethane. PA1 (1) impregnating a nonwoven fabric composed of a fiber which consists of at least two types of polymers with an aqueous polyurethane emulsion made of an aliphatic diisocyanate, polytetramethylene glycol and an aliphatic diamine; and PA1 (2) dissolving or decomposing part of the polymer of the fiber of the above impregnated fabric with an organic solvent or an aqueous alkaline solution. PA1 (1) impregnating a nonwoven sheet with an aqueous polyurethane emulsion mixed with inorganic salts, wherein said nonwoven sheet is composed substantially of a fiber layer made of microfine fibers of less than 0.5 denier, and said polyurethane emulsion has an average particle size of 0.1-2.0 .mu.m; and PA1 (2) heating the above impregnated sheet followed by drying. PA1 (i) said polyurethane emulsion is heat gellable type; and PA1 (ii) a film prepared by drying said polyurethane emusion at 50.degree. C. has a modulus of elasticity of 2.0.times.10.sup.7 to 5.0.times.10.sup.8 dyn/cm.sup.2 at 90.degree. C. at a thickness of 100 .mu.m. PA1 (1) impregnating a fibrous substrate made of microfine fiber-forming fibers with polyurethane emulsion (hereinafter called Polyurethane Emulsion (II)) meeting the conditions (i) to (iv) below; PA1 (2) coagulating the polyurethane; and PA1 (3) converting the microfine fiber-forming fibers to microfine fibers.
The method (1) described above tends to give a sheet of harder hand because the resin penetrates into microfine fiber bundles during the processing step and are held tightly in the fiber bundles. If the amount of the resin is reduced to improve softness, hand touch of the sheet becomes similar to that of the substrate itself lacking fulfillment feeling.
The softness may be improved by adding a softening agent after the fibrous substrate is impregnated. However, this requires an additional treatment step, lowers the productivity, and is unlikely to afford the desired high-grade hand touch similar to natural leather.
As for the method described above in (1), Japanese Kokai (laid-open) Publication H9-132876 proposes a process of manufacturing an artificial leather comprising the steps of:
Unfortunately, the sheet obtained by the process described above doe5 not provide adequate softness and fulfillment feeling desired for high quality artificial leather.
As for the method described above in (2), Japanese Kokai (laid open) Publication H6-316877, and Japanese Kokai (laid open) Publication H7-229071 disclose processes for making artificial leather comprising the steps of:
However, the process described above does not provide an artificial leather of enough softness and fulfillment feeling, failing to improve hand touch to a satisfactory level.
In order to ensure good quality, the wet coagulation process has been exclusively used in the industry to date to manufacture artificial leather even though it has lower productivity, and requires the use-of an organic solvent for the resin solution to be used for impregnation of the substrate.
However, from the standpoint of reducing solvent emission, improving safety in working environment, and streamlining the production processes, the use of the aqueous emulsion as described in the dry coagulation process is more preferable since the process does not require the use of solvent in impregnation and coagulation steps.
It is, therefore, strongly desired to develop a process capable of manufacturing high quality leather-like sheet having excellent softness and fulfillment feeling using an aqueous emulsion.